Polypropylene is conventionally used to produce fibers and spunbond nonwovens for a wide range of articles, such as, for example, disposable hygiene goods including diapers, sanitary napkins, training pants, adult incontinence products, hospital gowns, baby wipes, moist towelettes, cleaner cloths, and the like. The typical polypropylene nonwoven fabric can mimic the appearance, texture and strength of a woven fabric. In combination with other materials they provide a spectrum of products with diverse properties, and are also used alone or as components of apparel, home furnishings, health care, engineering, industrial and consumer goods. Conventionally, propylene based materials such as polypropylene that present excellent spinnability (e.g. stable fabrication without breaks of thin fibers on the order of about 0.7-2 denier and particularly about 1-1.5 denier) suffer from poor fiber and/or fabric properties (e.g. low tensile strength/tenacity). Inversely, polypropylene compositions that exhibit acceptable fiber/fabric properties such as good tensile strength have poor processability associated with fiber breaks and drips in the spinline, particularly when thin fibers are made (e.g. <20 microns or equivalently <2 denier). Thus, there is a general interest to impart superior tensile strength in both machine direction (MD) and transverse direction (TD, also referred to as Cross Direction, CD) of polypropylene nonwoven fabrics, while exhibiting excellent processability and spinnability, particularly for applications requiring improved mechanical strength such as disposable hygiene articles.
Likewise, in general, at low fabric basis weights (e.g. <15 g/m2), high line speeds (e.g. >600 m/min) and high throughput rates, conventional polypropylene resins do not provide the desired fabric strength properties. Thus, it is desirable to develop polypropylene fibers and fabrics that exhibit high fabric strength at low fabric basis weights and high line speeds. This allows the fabric converter to downgauge the spunbonding process utilizing less polypropylene resin (lower basis weight fabric) without sacrificing fabric mechanical properties. When used to prepare low basis weight (less than about 15 g/m2) spunbond fabrics at high line speeds (such as 900 m/min or more), typical polypropylene resins tend to show specific tensile strengths (tensile strength in N per 5 cm fabric width divided by fabric basis weight) of roughly 1 N/5 cm/gsm or less (where gsm is g/m2) in the transverse(cross) direction when run in a three beam spunbonding configuration.
Additional references of interest include: U.S. Pat. Nos. 7,105,603; 6,583,076; 5,723,217; 5,726,103; U.S. Patent Publication Nos. 2010/233927; 2011/059668; 2011/081817; 2012/0116338, 2010/0233928; 2008/0182940; 2008/0172840; 2009/0022956; PCT Publication Nos. WO 2010/087921; WO 2006/044083; WO 2006/118794; WO 2007/024447; WO 2005/111282; WO 2001/94462; JP 2007-023398 A (JAPAN POLYCHEM CORP, Feb. 1, 2007); and Journal Of Applied Polymer Science, John Wiley and Sons Inc., New York, May 2001, Vol. 80, No. 8, pp. 1243-1252. US2012-0116338A discloses spunbond fibers made from visbroken polypropylene with a melt flow rate of greater than 50 dg/min. US2010/0233928A discloses fabrics comprising fine meltspun fibers comprising one or more primary polypropylenes having a molecular weight distribution of less than 3.5 and a melt flow rate within the range from 5 to 500 dg/min, the fibers having at least one of an average diameter of less than 20 μm or a denier (g/9000 m) of less than 2.0.